The present invention relates to a process of manufacturing drawn tubing by turning a mandrel on its axis as well as to a drawn tubing manufacturing press of horizontal type.
Heretofore, there has been practised the so-called "push bench process" or a drawn tubing manufacturing process in which such a bottomed blank of the metal to be drawn as has been pierced in advance by the so-called "Ehrhardt piercing process" or a press piercing process is fitted on the mandrel of a horizontal press so that it may be drawn through a die having an orifice of the desired shape. The push bench process is accepted as one of the most important processes of manufacturing a seamless pipe because it is excellent for producing thick-walled tubing having a relatively large diameter.
For the drawing step, moreover, the horizontal press is used, the mandrel of which is secured to a cross head at all times during the production except when it is replaced by another for changing the pipe size, no matter whether the so-called "tandem die process" or "single die process" might be resorted to.
Here, in the push bench process of the above-specified kind, occurrence of irregularities in the thickness of the drawn product should be obviated as much as possible because the cutting allowance, which is taken into consideration when the drawing is to be finished is increased to not only elongate the tme period for the cutting step but also reduce the yield. Moreover, three major causes for inviting that irregular thickness have been found out along with auxiliary causes such as irregular size or burn of the formed blank. First of the major causes comes from the fact that application of a lubricant is limited to a portion of the surface of the blank, because the press is of the horizontal type so that the lubricant or glass powder is scattered only onto the upper surface of the blank. A second major cause is formed by the fact that a die may frequently be inclined with respect to the axis of the mandrel. A third major cause occurs as a result that the mandrel may be bent during or after the drawing step.
By those three major causes and the auxiliary causes thus far described, the thickness of the drawn tubular product becomes longitudinally and circumferentially irregular. Noting that the occurrence of the thickness irregularities is strongly dependent upon that the working conditions are different in the circumferential direction, we, the Inventors, have succeeded in discovering the fact that the drawn tubing having a uniform thickness can be finally produced if the working conditions are satisfied to automatically correct the thickness irregularities which have been caused at a preceding drawing step. Specifically, since a plurality of drawing steps are incorporated in the usual push bench process, the satisfaction of those working conditions can be attained if a subsequent drawing step is conducted after the mandrel has been turned a predetermined abgle, e.g., 180 degrees at an interval during two consecutive drawing steps.